Electrical computers: arithmetic processing and calculating – Electrical digital calculating computer – Particular function performed
Reexamination Certificate
2001-05-15
2004-05-11
Malzahn, David H. (Department: 2124)
Electrical computers: arithmetic processing and calculating
Electrical digital calculating computer
Particular function performed
C708S256000
Reexamination Certificate
active
06735606
ABSTRACT:
BACKGROUND
1. Field
The present invention relates generally to communications, and more specifically to a novel and improved method and apparatus for generating a pseudorandom noise (PN) sequence composed of one or more PN sequences, with the ability to rapidly slew from one offset to another.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98 standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (4) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (5) some other standards. These named standards are incorporated herein by reference. A system that implements the High Rate Packet Data specification of the cdma2000 standard is referred to herein as a high data rate (HDR) system. The HDR system is documented in TIA/EIA-IS-856, “CDMA2000 High Rate Packet Data Air Interface Specification”, and incorporated herein by reference. Proposed wireless systems also provide a combination of HDR and low data rate services (such as voice and fax services) using a single air interface.
Pseudorandom noise (PN) sequences are commonly used in CDMA systems for spreading of transmitted data, including transmitted pilot signals. CDMA receivers commonly employ RAKE receivers. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilots from neighboring base stations, and two or more multipath demodulators (fingers) for receiving and combining information signals from those base stations.
Inherent in the design of direct sequence CDMA systems is the requirement that a receiver must align its PN sequences to those of the base station. The time required to transmit a single value of the PN sequence is known as a chip, and the rate at which the chips vary is known as the chip rate. For example, in IS-95, each base station and subscriber unit uses the exact same PN sequences. A base station distinguishes itself from other base stations by inserting a unique time offset in the generation of its PN sequences. In IS-95 systems, all base stations are offset by an integer multiple of 64 chips. A subscriber unit communicates with a base station by assigning at least one finger to that base station. An assigned finger must insert the appropriate offset into its PN sequence in order to communicate with that base station. It is also possible to differentiate base stations by using unique PN sequences for each rather than offsets of the same PN sequence. In this case, fingers would adjust their PN generators to produce the appropriate PN sequence for the base station to which it is assigned. Adjusting the offset in the PN sequence is known as slewing.
An early CDMA PN generator commonly consisted of a linear feedback shift register (LFSR). When not slewing, the LFSR would be enabled once per chip to produce a new state and output a new chip in the PN sequence. To perform slewing, the LFSR would be either disabled to perform a retard, or enabled twice per chip to perform an advance. Thus, a simple PN generator might be capable of stewing in either direction at a rate of one chip per chip time. A slight improvement can be had if the clock rate of the LFSR is a higher rate, for example eight times the chip rate. Then, advances could be performed at a rate of eight chips per chip time (since the enable could be activated for all eight cycles occurring in a chip). Retards would still be limited to one per chip. Control logic can be added to such an LFSR based system such that slewing can be directed by simply providing an offset to the PN generator and a command to slew to that offset. While slewing in an early PN generator may have been performed by continuously moving the PN state, as just described, the term “slewing” is used generally throughout the following description to identify any process of producing a desired offset in a PN sequence.
Overall system performance is enhanced when each finger can rapidly align its PN sequence with the transmitted PN sequence. There are a variety of reasons for this. Upon initial acquisition, a fast slewing PN generator will reduce the time from finger assignment to demodulation. A searcher, so equipped, will be able to locate neighboring base stations sooner, and thus handoff will be more efficient and effective. Strong multipath signals that fade in and out rapidly are more likely to be demodulated and usefully combined when fingers can respond rapidly to changes in PN offset. Therefore, it is desirable to utilize PN sequences which can rapidly transition from one offset in a PN sequence to another.
One such PN generator is disclosed in U.S. Pat. No. 6,154,101 entitled “FAST SLEWING PSEUDORANDOM NOISE SEQUENCE GENERATOR”, assigned to the assignee of the present invention. This PN generator provides rapid slewing for PN sequences generated from a single linear feedback shift register (LFSR), such as those required for IS-95 and similar systems. This PN generator contains an LFSR and a reference counter, each of which is loadable. A free-running counter is used to maintain a reference time and the desired offset is added to that reference time to provide a target location. A look-up table is then accessed to find the PN state and PN count corresponding to the target location. If the table is fully populated, then the PN count is simply the target location, and the associated PN state is retrieved. If the table is not fully populated, the target location may not exist in the table. In this case, the closest PN count value with an associated PN state is located. The PN state and the PN count are then loaded simultaneously into the LFSR and the reference counter. Hence, the PN generator has now instantaneously slewed to the offset given by the difference between the free-running counter and the PN count value. In IS-95, the I and Q PN sequences are generating from a single LFSR using different masks. Thus, this technique inherently keeps the I and Q PN sequences aligned.
There may be some residual slewing required to get the PN generator exactly to the desired offset given by the target location. One reason for this is that, as stated above, the look-up table may contain only a subset of the possible PN states, and so the instantaneous-load slew only gets close to the target. Another reason is that there may be a slightly variable time delay between reading the free-running counter, accessing the look-up table, and loading the results. This residual slewing can be accomplished with the traditional slewing methods described previously.
The previously described technique works excellently for PN sequences that can be generated using a single LFSR. There are other classes of PN sequences which themselves are generated from other PN sequences, such as Gold codes. The W-CDMA standard is an example of a CDMA system which uses Gold codes for I and Q PN spreading. A Gold code is generated by summing (XORing) the output of two LFSRs.
Agrawal Avneesh
Terasawa Daisuke
Venkatesan Sivarama
Brown Charles D.
Malzahn David H.
Pappas Geroge C.
Qualcomm Incorporated
Wadsworth Philip R.
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